Thanks for your answer, Mende!
Do you have any info how much wider the IMSA gears are over the street turbo gears and how they managed to fit it inside the basically same gearbox space?

I actually expected that the IMSA would have a Colotti box, like the GTV6 Gr.A, but not according to your experience. Well, they have problems with Colotti boxes too.

Also about the propeller shaft of this car: How much of the rubber donuts were replaced by universal joints. Just the front or all of them? Any photos of how the propeller shaft looks on the IMSA turbo?

Thanks for your answer, Mende!
Do you have any info how much wider the IMSA gears are over the street turbo gears and how they managed to fit it inside the basically same gearbox space?

I actually expected that the IMSA would have a Colotti box, like the GTV6 Gr.A, but not according to your experience. Well, they have problems with Colotti boxes too.

Also about the propeller shaft of this car: How much of the rubber donuts were replaced by universal joints. Just the front or all of them? Any photos of how the propeller shaft looks on the IMSA turbo?

G.K.

The gears are app. 2-3 mm wider.

I cannot answer you if IMSA 75's did have a colotti or not, it is very difficult to find any history on these cars.

The prop shaft is with 2 CV joins and one rubber dount. See att. pics. But, I can not say 100 % sure that this prop shaft is exaclty as they looked like at that time. If anybody knows it would be great full.

My IMSA cars are not original, none of them. But many of the parts are ogiginal IMSA parts.

Thanks for interesting photos, Mende!
Any of the propeller shaft parts available from shelf or on order?

You mentioned that there was an oil pump in the box. Makes me remember that the SZ had one too. Also the SZ prototypes used IMSA technology as a starting point, got it detuned and adapted for road use. Could it be that the SZ had some parts of this gearbox and also was strenghtened with wider gears? A way to find it out would be to compare spare parts numbers of 75 3L and the SZ...but maybe somebody knows already?

Here the oilpump installation on the SZ. This box is derived from the IMSA. How much IMSA is left could be something to find out! Can you find similarities with your own IMSA box?

I can help if your after racing parts for your car, I have Uj'd propshafts, strengthened clutch housings, close ratio straight cut gear kits and other stuff.
Its interesting to note that the dogbox pictured run a wide tooth helical gear rather than a straight cut.
I have an sz gearbox That I aquired from a friend and the gear box is totally standard apart from the final drive being a differrent ratio to the usual 3.9 in a 3l box.
I cant post pictures of the propshafts due to size so if yiour interested Pm me and I will pass on pictures.
I have to say that is quite some IMSA 75!

helical gears are stronger then straight cut due to more contact area.
The down side is they cost a lot more to make and they put a lot of thrust loading on the box pushing it apart. One of the problems with the alfa box it is seem to not be able to take the thrust and flexes then the gears bind and boom.

I even wonder if that happens when the pinon breaks if it is not due to the box flex making the mesh wrong.

Your right that flex can cause the mesh problems on the crown and pinion usually if you jump kerbs alot, the shock just kills them! but i would argue the helical gears beings stronger, The contact patch has little to do with it. I have freinds that produce helicopter gear boxes who say straight cut is the way to go for strength and reliability. you get less power loss so you transmit more power which is the goal of any system.
I am not saying that helical is bad at all just there are advantages and disadvantage to any system. Most race boxes I have seen and have are straight cut other than a few bacchi boxes that are wide tooth helical.
I would love to develop a dog box but I wonder what the maintenance costs are compared to a standard synchro and if the advantage id worthwhile for the average racer?

going dog should be easy on a alfa box as they are one of the very few boxes that the dogs come off the gears. but there is the slop in a dog box it would be a pain on the street. when going a constant speed it would go bang bang every time the motor goes from pushing to pulling as the slack is taking up.
They work great on the racy way as you seem to be WOT or all the way off the gas.

I think why most race boxes use straight cut is that they are super easy to make. cutting a good helical is very hard and takes a lot of time. it is also a lot harder to design. almost any cad program can do straight cut design. and the gears can be cut on a cheap 3 axis mill. so if the gear can cost 10x less and can be cut way faster what is a little noise on a race car.
If you went helical there is more risk that there is a error in the design. you have to wait a real long time for them as they take a long time to cut and they cost a whole lot more. and what is gained on a race car, quiet? the strength is more or less a wash if using a stock box that the housing flex might be the limiter.

Race gearboxes use straight cut gears simply for lack of power loss. With a helical gear there is more rubbing as the teeth engage, this robs power.

But a helical gear is stronger, all other things being equal because the tooth is longer and thus has more steel supporting it.

I find it amusing (?) that the 75 gearbox is considered weak, particularly because I've asked why at least once and received no answer, ie. is it the gears or the casing, or what? A few posts back now hint that it is the casing allowing flex and causing the pinion and ring to not mesh correctly.

Surely this can be fixed, by reinforcing ... and would be far cheaper than purchasing Maserati gearboxes that would suffer road car ratios ???

We really need a thread that discusses each failure or weakness so we can work on a resolution ... otherwise these gearboxes will soon become scarce and all these 75's being tracked all over the world will have to retire .
Pete

look at the posts by Barry over at the alfagtv6.com,
I think he had poped more boxes then anyone and had done all of the above.
I would say he is the go to guy for beefing up a alfa box.

but I think at the end of the day the box is kinda balanced. fix one problem and a different one comes up. but I think the flex might be chronic and reinforcing can only do so much. at lest with out casting a new case. but that is just my guess.

We really need a thread that discusses each failure or weakness so we can work on a resolution ... otherwise these gearboxes will soon become scarce and all these 75's being tracked all over the world will have to retire .
Pete

Hi,

Thats why I here try an other angle by asking an owner of an IMSA car what modification is on the drivetrain of that which is already proved of being capable of handling 400hp and more. Instead of trial and failing as many have tried why not get the recepe from a developed product and make a similar solution.
A debate could be alright of course to clarify the problem points. However personally I would like to advance further beyond debate point and install the parts which make the job and get on with life. Hopefully a manufacturer of the right parts will turn up!

Thats why I here try an other angle by asking an owner of an IMSA car what modification is on the drivetrain of that which is already proved of being capable of handling 400hp and more. Instead of trial and failing as many have tried why not get the recepe from a developed product and make a similar solution.
A debate could be alright of course to clarify the problem points. However personally I would like to advance further beyond debate point and install the parts which make the job and get on with life. Hopefully a manufacturer of the right parts will turn up!

G.K.

We developed a new dog gear box with lower gear ratios mainly for being able to run with the stronger crown pinion f.ex. 10/39 instead of 10/43 and without having an unusable top speed. The crown and pinion is the weakest part of the alfa gear box when going up in engine power. I will post pictures of the new box soon.

I've broken two gearboxes, due to the spacer in between the 2 halves of the gearbox. The spacer holds the bearings for the shafts and sometimes parts of it chip off and drop stuff in the gearbox casing. Did you guys also strengthen that part?

I've broken two gearboxes, due to the spacer in between the 2 halves of the gearbox. The spacer holds the bearings for the shafts and sometimes parts of it chip off and drop stuff in the gearbox casing. Did you guys also strengthen that part?

This part you think on, is it the shims to allign crown and pinion?
If so, I have not had any issues with this.
Also on the picture below here you can see how we do allignement of crown and pinion.

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